I worked both as a reporter and in public relations (not at the same time) and stuff like this happens for various reasons:

  - an institution can publish a report as part of a regular schedule (e.g. unemployment by the BLS) or as one shot thing (e.g. a study on clowns distribution in arid areas). This leads many reporters to publish articles about basically the same subject, but in an uncoordinated manner;  

 - PR agencies often coordinate with media outlets from various backgrounds and markets to publish about some particular topic (company, product, campaign, ...) either at the same time, or in coordinated waves;  

 - trends and public discourse can make it so that many sources cover almost the same thing at the same time (e.g.: bitch resting face, rat boy summer, ...);  

 - luck is, always, a factor.